The present invention relates to supports for carrying integrated circuit chips.
In the well-known technique of producing integrated circuits, a disc or a wafer cut out from a monocrystal of a semiconductor material such as silicon is subjected to a series of operations consisting of doping, masking, photo-etching, and the ionic diffusion or injection of doping agents, so as to form within the disc a plurality of identical, regularly distributed integrated circuits. The disc is then coated with a protective layer of an insulator such as glass and is subsequently cut up into tiny plates which are generally referred to as "chips", each chip containing one complete set of integrated circuits. Each chip is provided with contact areas which are laid out in a predetermined configuration and which, by means of interface conductors soldered to them, are intended to provide the electrical connections between each chip and an interconnecting base such as a printed circuit board.
The chips are very small in size and in order to make them easier to handle and to facilitate their mounting on an interconnecting base, it has been proposed that the chips should be attached to a flexible strip of an inextensible insulating material which, to allow it to be moved along, is provided with regularly spaced lateral perforations and, to allow the chips to be mounted, has equidistant openings towards the centre of each of which extend overhanging interface conductors.
Reference herein to the "centre" of an opening provided in the flexible strip support is not intended to refer to the geometric centre of the opening formed by the intersection of its axis. Rather, reference to the "centre" of an opening as used throughout the specification and claims is intended as a means of general orientation to define an area which is centred with respect to the sides of the opening.
The overhanging interface conductors are cut from a film of conductive material which is formed on one of the faces of the strip. The arrangement of the free inner ends of the interface conductors in each opening corresponds to that of the contact areas on a chip which is placed in the centre of the opening. This being the case, the chips are mounted on the strip by positioning them under respective ones of the openings in the strip, in such a way that the contact areas on the chip line up with the inner ends of respective interface conductors associated with the opening and by then soldering these ends to the said contact areas.
The chips which have been mounted on the strip in this way may then be attached to an interconnecting base such as a printed circuit board. To this end, the interface conductors whose inner ends are soldered to the contact areas on a chip are cut at a short distance from the edges of the chip. The chip, having been separated from the strip in this way, is then soldered by its non-working face, i.e. the face which has no contact areas, to a die bonding pad on the interconnecting base. After this, the ends of the parts of the interface conductors which remain projecting from the chip are bent so that they can be brought into contact with the corresponding conductive areas on the interconnecting base and can be soldered to them. In the course of this operation, these portions of the interface conductors are subjected to deformation and this, particularly in cases where the contact areas are relatively far away from the edges of the chip, may bring them into contact with the said edges. If the protective insulating layer covering the working face of the chip has peeled away along the edges of the chip when the disc was being cut up, a short circuit may then occur between the uninsulated edge of the chip and the part of the conductor brought into contact with it.
To overcome this drawback, a method has been proposed in the prior art which consisted in forming on the working face of the disc, before it was covered with the insulating protective layer, U-shaped dividing grooves whose width is greater than that of the saw cuts made when the disc was cut into chips. The result was that the upper edges of the chips produced by the cutting operation had a kind of bevelled appearance, which reduced the risk of part of a conductor making contact with one of the edges of the chip to which it had been soldered. However, such a method has other drawbacks. In effect, even with bevelled edges it is difficult, when the disc is cut up, to prevent particles of conductive material from being cast onto the surface of the protective layer and thus, by combining together, forming a deposit which, in the end, entirely nullifies the insulating effect of the protective layer. Furthermore, given on the one hand that the insulating material which is applied to the disc after the formation of the dividing grooves builds up in the grooves to a lesser extent than on other parts of the disc, and on the other hand that the thickness of the insulating layer is necessarily limited by the height of the contact areas projecting from the working face of the disc, it is very difficult to provide effective protection for the edges of the chips formed by cutting up the disc, and thus to remove the risk of short circuits along these edges.
This is why, so as to avoid all these disadvantages, it has been found preferable before mounting the chips on the strip, to arch the interface conductors on the strip, that is to say to deform them relative to the plane of the strip in such a way that it is not possible for them to touch the edges of the chip onto which they are then soldered, even in the cases where after they have been cut, the free ends of their remaining parts are bent so as to be soldered to the conductive areas on the interconnecting base. However, this procedure creates difficulties. It was in fact found not only that the arched shape was altered as a result of the interface conductors being soldered to the contact areas on the chips, but also that interface conductors so arched acquired greater flexibility in the plane of the strip, which caused them to distort in this plane at the moment when their free inner ends were pressed against the contact areas projecting from the working faces of the chips. In cases where these areas were very close together, this meant that there was a danger of each interface conductor slipping on the area against which it was pressed and making contact with a neighboring contact area thus short-circuiting the two areas concerned. To overcome this disadvantage it was thought to be enough to arch the interface conductors only after they had already been soldered to the contact areas on the chips. Unfortunately, the results achieved with this procedure have so far been disappointing, due to the fact that it usually results in the interface conductors breaking or becoming unsoldered.